Processes for the production of various C.sub.8 alkyl isomers are of importance within the petroleum and petrochemical industries. This interest is a result of the demand for specific isomers, in particular, p-xylene and o-xylene. P-xylene is a valuable chemical feedstock which may be used in the synthesis of polyesters. The p-xylene may be derived from mixtures of C.sub.8 alkyl aromatics separated from such raw materials as petroleum napthas and pyrolysis distillates, usually by selective solvent extraction. The C.sub.8 alkyl aromatics in such mixtures and their properties are as follows:
Freezing Boiling Density Point (.degree. F.) Point (.degree. F.) (lbs/U.S. gallon) Ethylbenzene -139.0 277.0 7.26 p-xylene 55.9 281.0 7.21 m-xylene -54.2 282.4 7.23 o-xylene -13.3 292.0 7.37
The C.sub.8 alkyl aromatic fractions from the above identified sources may vary quite widely in composition, but usually comprise about 10-32 weight percent ethylbenzene, with the balance, xylenes, being divided approximately as 50 weight m-xylene and 25 weight percent each of p-xylene and o-xylene.
Thus, a mixed C.sub.8 alkyl aromatic stream may be fed to one or more separation steps or units, e.g., crystallization, adsorption, superfractionation and the like, for separation of one or more specific C.sub.8 alkyl aromatic isomers. The remaining C.sub.8 alkyl aromatic material is often fed to an isomerization reaction zone wherein the concentration of the desired isomer or isomers is replenished. The effluent, or at least a portion of the effluent, from the isomerization reaction zone is then fed to the separation unit for recovery of the desired isomer or isomers.
In one particular C.sub.8 alkyl aromatic isomerization process, the isomerization is commonly affected by contacting the hydrocarbon in admixture with hydrogen at isomerization conditions with a dual function catalyst possessing both hydrogenation and cracking activities thereby effecting the desired isomerization reaction. By contacting the C.sub.8 alkyl aromatic with the catalyst at isomerization conditions, C.sub.8 naphthalenes, toluene and C.sub.9 + aromatics, among other by-products, are often produced. It has been found that such C.sub.8 naphthenes are beneficially maintained in the C.sub.8 alkyl aromatic stream sent to the separation zone and then recycled back to the isomerization zone to improve the yield of the specific or desired C.sub.8 aromatic isomer. A number of patents have disclosed processing schemes to take advantage of the beneficial effects of recycling C.sub.8 naphthenes to a C.sub.8 alkyl aromatic isomerization zone. Among these patents are U.S. Pat. Nos. 3,538,173 and 3,553,276.
Although many prior art processes have talked in terms of providing improved yields of the specific or desired alkyl aromatic isomer, it would be clearly advantageous to provide still further improvement in C.sub.8 alkyl aromatic isomer yields. Therefore, one of the objects of the present invention is to provide an improved process for isomerizing a C.sub.8 alkyl aromatic hydrocarbon-containing feedstock. Another object of the present invention is to provide a process for maintaining the positive activity of a catalyst to isomerize a mixed composition of C.sub.8 alkyl aromatics isomers to one or more desired isomers. A still further object of the present invention is to provide a C.sub.8 alkyl aromatic isomerization process with reduced xylene ring loss. Yet another object of the present invention is to provide a process for converting ethylbenzene to xylene with reduced xylene ring loss by limiting the temperature of the reduction step in the activation of the catalyst to no more than 340 degrees celsius. Yet another object of the present invention is to provide an improved process for isomerizing a C.sub.8 alkyl aromatic hydrocarbon-containing feedstock that is simple and economical to operate.
The present invention is improvement to the xylene isomerization process known as "Octafining II" and described in U.S. Pat. No. 4,139,571. Briefly, the Octafining II process employs a noble metal catalyst in combination with a p-xylene recovery process and/or o-xylene recovery by fractionation.
Further features and other objects and advantages of the present invention will become apparent from the following detailed description.